(1) Field of the Invention
The present invention relates to a secondary cell employing a conductive polymer as at least a positive electrode.
(2) Description of the Prior Art
In recent years, as disclosed in Japanese Patent Publication Kokai No. 56-136469, a secondary cell employing a conductive polymer as an electrode has been proposed.
A conductive polymer employed for this type of secondary cell, in general, has a poor conductivity. However, the polymer can be doped or undoped with various dopants, which improves the conductivity of the polymer remarkably. The polymer doped with an anion such as ClO.sub.4 - and BF.sub.4 - is employed as a positive electrode material and the polymer doped with a cation such as Li .sup.+ and Na.sup.+ as a negative electrode material, respectively. Then, a rechargeable cell is produced by effecting doping and undoping electrochemically and reversibly.
The conductive polymer is generally produced by means of chemical polymerization or electrolytic polymerization by an oxidizing agent and, polyacetylene, polypyrrole, polythiophene, polyaniline and polyparaphenylene have been known as representive examples. If the polymer is obtained in powdery form it is molded under pressure for use and if in film form, it is punched to the shape corresponding to an electrode or reduced to powder. A cell employing a conductive polymer thus obtained is light in weight and has a high energy density. Further, hopes are placed on this type of cell because of its characteristics of being pollution-free.
Particularly, polypyrrole and polyaniline described above have good characteristics, and therefore cells employing these materials have bright prospects for practical use.
Usually, an electrolyte employed in this type of secondary cell is prepared by dissolving alkaline metallic salt such as lithium salt, for instance, lithium perchlorate and lithium tetrafluoroborate, in an organic aprotic solvent such as propylene carbonate. This electrolyte has already been used in nonaqueous cell such as a lithium cell.
However, a secondary cell employing the conductive polymer as an electrode material has a higher electrode potential than an existing nonaqueous cell. Accordingly, when a cell comprising the above conventional electrolyte is charged and discharged the cell voltage becomes excessively high with progress of the charging.
The charge end voltages of lithium secondary cells emoploying V.sub.2 O.sub.5, polypyrrole and polyaniline as a positive electrode material were examined. The results are shown in the following Table 1. The cells were charged with a current of 1mA for 10 hours and the electrolyte of each cell comprised a solution obtained by dissolving 1M of lithium perchrolate in propylene carbonate.
TABLE 1 ______________________________________ positive electrode charge end voltage (V) ______________________________________ V.sub.2 O.sub.5 3.19 RuO.sub.2 2.72 WO.sub.3 2.06 MoO.sub.2 1.75 Nb.sub.2 O.sub.5 1.76 TiS.sub.2 2.88 NbS.sub.2 2.97 polypyrrole 4.30 polyaniline 4.35 ______________________________________
The above results show that the cells employing polypyrrole and polyaniline as a positive electrode material have high charge end voltages compared with the cells employing the other materials(V.sub.2 O.sub.5 and so on).
As a result, the former cells have the problems of lowering charge and discharge efficiency and deteriorating storage characteristics due to the side reaction wherein the electrolyte, dopant and conductive polymer are decomposed. The larger charging capacity the cell has, the more remarkable this tendency is. Therefore, in such a case, deterioration in cycle characteristics is so serious that the cycle life of the cell becomes short.
The following Table 2 shows, by way of reference, the operating voltages of the lithium primary cells employing various positive electrode materials.
TABLE 2 ______________________________________ positive electrode operating voltage (V) ______________________________________ MnO.sub.2 3.0 CF 2.6 SO.sub.2 2.8 Ag.sub.2 CrO.sub.4 3.0 CuS 2.2 FeS.sub.2 1.6 ______________________________________
As apparent from the above table, the operating voltages of the lithium primary cells are not so high as to cause the conventional solvents to be decomposed. Accordingly, the above problems do not rise where MnO.sub.2 or the like is used as a positive electrode material.